1. Field of the Invention
The present invention relates to a process for casting polycrystalline silicon blocks as a base material for solar cells, in which introduced solid silicon is gradually fused by supplying heat in a melting station, released from there continuously or in phases, fed to a mold and caused to solidify therein, and to equipment for carrying out the process.
2. The Prior Art
Such a process is known from European Patent No. 55,310. In this process, as also in other hitherto known processes, molten silicon is cast to give polycrystalline blocks (see, U.S. Pat. No. 4,382,838) which are eventually sawn into disks, and if appropriate, separating off unsuitable edge regions and severing into smaller blocks. They are subsequently further processed to yield solar cells. The mold is always completely filled with the silicon melt and the actual solidification step is only then initiated, if appropriate, after transfer to a separate crystallization station. It is typical of the blocks obtained that, in most cases, an approximately vertical alignment of the monocrystalline regions can be observed only in the center, whereas the orientation in the direction of the edge regions parabolically approaches the horizontal. The consequences of this crystallization characteristic are, on the one hand, a greater number of crystal defects in the crystallites and, on the other hand, unfavorable segregation effects which lead to an enrichment of residual impurities in the interior of the blocks. Both these effects, however, shorten the diffusion lengths of the minority carriers and, thus, reduce in the end the efficiency of the solar cells obtained.
Ever since the technology of casting silicon blocks as a base material for solar cells began (see, for example, German Patent No. 2,508,803, and the corresponding U.S. Pat. No. 4,382,838), the object was, therefore, to arrange the crystallization of the molten silicon in the mold in such a way that, as much as possible, vertical crystalline growth starting from the mold bottom occurs, and the so-called "edge growth" starting from the mold walls is suppressed as much as possible. Such a process method, which greatly reduces the heat flux via the side walls of the mold, results, however, in long contact times between the mold wall and the melt, which leads to an enrichment of impurities in the block material.
Another attempted solution practiced in a number of the known casting processes, therefore, aims for rapid solidification of the melt in the entire edge region, i.e., especially from the side walls of the mold (see, for example, German Offenlegungschrift No. 3,427,465 and the corresponding U.S. Pat. No. 4,769,107). In this case, however, increased edge growth must be accepted in any case, even if attempts are made by additional heating measures to slow down this growth significantly in the interior of the blocks.